Organic electrolyte solution type cell

ABSTRACT

An organic electrolytic solution type cell using an electrolytic solution of a lithium salt in a polar solvent which comprises a compound having an organophobic group and an organophilic group, which cell has improved storage stability and good charge-discharge characteristics.

This application is a continuation of Ser. No. 738,607 filed on Jul. 31,1991, now abandoned, which is a divisional of Ser. No. 499,667 filed onMar. 27, 1990, now U.S. Pat. No. 5,085,594, the entire contents of whichis incorporated by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an organic electrolytic solution typecell comprising a non-aqueous electrolytic solution such as a lithiumcell.

2. Description of the Related Art

As the non-aqueous electrolytic solution for an organic electrolyticsolution type cell, a solution of a lithium salt in a polar solvent isused. Examples of the lithium salt are LiClO₄, LiAsF₆ and LiPF₆, andexamples of the polar solvent are propylene carbonate, γ-butyrolactone,dimethoxyethane and dioxolane.

In the cell comprising the non-aqueous electrolytic solution, a negativeelectrode Li reacts with a harmful material such as water, oxygen gas,nitrogen gas and impurities which are contained in the electrolyticsolution or, in some case, components of the electrolytic solution, sothat the surface of the lithium electrode is deactivated. Thus, cellperformance, such as closed circuit voltage, are deteriorates duringstorage.

When the organic electrolytic solution type cell is used as a secondarycell, the reaction between the harmful materials in the electrolyticsolution and the lithium electrode deteriorates the charge-dischargecycle characteristics.

LiPF₆ is practically preferable as an electrolyte since it is safe,unlike LiClO₄, or it is a toxic, unlike LiAsF₆. However, it is thermallyunstable and decomposes during storage at a high temperature. Therefore,the cell performance is further deteriorated.

To improve the thermal stability of LiPF₆, it is proposed to addhexamethylphosphoric triamide (hereinafter referred to as "HMPA") ortetramethylethylenediamine (hereinafter referred to as "TMEDA"). Sincesuch additives do not achieve sufficient effects and HMPA may react withactive lithium metal of the negative electrode, improvement of thestorage stability cannot be expected.

SUMMARY OF THE INVENTION

One object of the present invention is to provide an organicelectrolytic solution type cell having improved storage stability andcharge-discharge cycle characteristics.

Another object of the present invention is to provide an organicelectrolytic solution type cell having thermal stability even when LiPF₆is used as the electrolyte.

These and other objects of the present invention are achieved by anorganic electrolytic solution type cell using an electrolytic solutionof a lithium salt in a polar solvent which comprises a compound havingan organophobic group and an organophilic group.

With the compound of the polar solvent having the organophobic group andthe organophilic group, the surface of the lithium electrode may beprotected so that the reaction of lithium metal with the electrolyticsolution, impurities or water may be prevented.

In a preferred embodiment of the present invention, a trialkylamine ofthe formula: ##STR1## wherein R₁, R₂ and R₃ are the same or differentand each is an alkyl group having at least 3 carbon atoms at least oneof hydrogen atoms of which may be substituted with a fluorine atom isused in combination with LiPF₆ as the electrolyte. In such acombination, the trialkylamine (I) not only protects the lithiumelectrode, but also stabilizes the LiPF₆ so that the cell has goodstorage stability. In addition, when the trialkylamine (I) is usedtogether with other stabilizers for LiPF₆, the storage stability of thecell can be further improved.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 schematically shows the function of the compound having theorganophobic group and the organophilic group in the organicelectrolytic solution type cell,

FIG. 2 is a cross sectional view of one embodiment of the organicelectrolytic solution type cell of the present invention, and

FIG. 3 is a graph showing the results of stability tests of the cells inthe Examples and the Comparative Examples.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 shows a model of the function of the compound having theorganophobic group and the organophilic group in the organicelectrolytic solution type cell. Since the organophobic group of thecompound is very slightly soluble in the electrolytic solution, it facesthe lithium electrode surface, while the organophilic group of thecompound, which is soluble in the electrolytic solution, faces theelectrolytic solution. Due to such properties of the organophobic andorganophilic groups of the compound, the molecules of the compound arearranged and orientated as shown in FIG. 1. As a result, the reactionbetween the lithium electrode surface and the harmful materials isprevented.

The molecules of the compound do not greatly suppress liberation of thelithium ions (Li⁺) during discharge of the cell, since the molecules arepresent on the lithium electrode surface in the form of a singlemolecule layer and the lithium ions can be liberated into theelectrolytic solution via the organophilic groups.

In the present specification, the organophobic group is intended to meana part of an organic molecule, which part has low solubility in thepolar solvent.

The organophobic group of the compound is preferably an alkyl group atleast one hydrogen atom of which may be substituted with a fluorineatom. Preferably, the alkyl group is a straight chain or linear one. Toimpart organophobicity to the group, the alkyl group has at least 3carbon atoms, preferably at least 4 carbon atoms. Preferably, the numberof the carbon atoms in the alkyl group does not exceed 10.

The organophilic group is intended to comprise a part of an organicmolecule, which part has a high solubility in the polar solvent.Preferred examples of the organophilic group are an amino group, aketone group, an ether group and an ester group. The amino group haslarge affinity with the lithium ions and large solubility in thesolvent. The ketone group and the ester group have relatively largeorganophilicity and less reactivity with the lithium metal. The ethergroup has adequate organophilicity and is most stable against thelithium metal. Usually, one organophilic group is bonded with at leasttwo organophobic groups. The combination of the organophobic group andthe organophilic group and the amount of the compound to be added to theelectrolytic solution are determined according to the functions of thesegroups.

The present invention is now explained in detail by using thetrialkylamine as an example of the compound having the organophobicgroup and the organophilic group.

The trialkylamine is represented by the formula (I). The number of thecarbon atoms in R₁, R₂ or R₃ is at least 3, preferably at least 4.Examples of the trialkylamine are tributylamine, trihexylamine andtridecylamine.

The content of the trialkylamine in the electrolytic solution isgenerally from 0.05 to 5% by volume, and preferably from 0.1 to 1.5% byvolume. When the content of the alkylamine is too small, the intendedeffects are not achieved. When the content is too large, the cellperformance such as the closed circuit voltage (CCV) of the cell duringdischarge is decreased.

The trialkylamine not only protects the lithium electrode surface butalso stabilizes LiPF₆. Therefore, the trialkylamine is preferably usedwhen the electrolytic solution contains LiPF₆.

Together with the trialkylamine, other stabilizers for LiPF₆ may beused. In this case, the lithium electrode surface is protected and thestabilizing effect of the trialkylamine is strengthened.

Examples of the other stabilizers are a compound having a bond of theformula: >N--P(═O) (e.g. HMPA), tetraalkyldiamines (e.g. TMEDA) andpyridines. Preferably, an N-dialkylamide of the formula: ##STR2##wherein R₄ or R₅ are the same and different and each being a saturatedhydrocarbon group having from 1 to 10 carbon atoms, preferably 1 to 3carbon atoms, and optionally an oxygen atom or a nitrogen atom in acarbon chain, and R₆ is a hydrogen atom or the same hydrocarbon group asabove, provided that two of the R₄, R₅ and R₆ may together form a ring.

The N-dialkylamide (II) strongly stabilizes LiPF₆ and hardly reacts withthe lithium negative electrode.

Specific examples of the N-dialkylamide (II) are 1-methyl-2-piperidone,1-methyl-2-pyrrolidinone, 1-ethyl-2-pyrrolidinone,N,N-dimethylacetamide, N,N-diethylacetamide, N,N-dimethylformamide,N,N-dimethylpropionamide, 1,5-dimethyl-2-pyrrolidinone,1,3-dimethyl-3,4,5,6-tetrahydro-2(1H)-pyrimidinone, 4-formylmorpholine,1-formylpiperidine, 1-(3-methylbutyryl)pyrrolidine, N-methylcaprolactam,bispentamethyleneurea, 1-cyclohexyl-2-pyrrolidinone,N,N-dimethyldodecaneamide, N,N-diethylformamide,N,N-diethylpropioneamide, 1,3-dimethyl-2-imidazolidinone and the like.

The amount of the stabilizer for LiPF₆ in the electrolytic solution maybe from 0.1 to 5% by volume, preferably from 0.2 to 1.5% by volume, anda total amount of the trialkylamine (I) and the stabilizer is generallyfrom 0.1 to 10% by volume, preferably from 0.2 to 5% by volume.

Examples of the polar solvent are propylene carbonate, γ-butyrolactone,dimethylsulfoxide, ethylene carbonate, 1,2-dimethoxyethane,tetrahydrofuran, 1,3-dioxolane, 4-methyl-1,3-dioxolane,2-methyltetrahydrofuran, other aliphatic monoethers and polyethers andthe like.

Examples of the electrolyte are LiPF₆, LiClO₄, LiCF₃ SO₃, LiBF₄, LiCF₃CO₂, LiAsF₆, LiB(C₆ H₅)₄, LiSbF₆ and the like. They may be usedindependently or as a mixture of two or more of them.

The amount of the electrolyte is selected from the range of 0.2 to 1.5mole/liter so that conductivity of the electrolytic solution is at least3 ms/cm at 25° C. according to the kind of electrolyte.

The organic electrolytic solution type cell of the present inventionuses the non-aqueous electrolytic solution of the electrolyte in thepolar solvent to which the compound having the organophobic group andthe organophilic group, such as the trialkylamine (I), is added, andincludes various primary and secondary cells.

Examples of active materials for the positive electrode are metal oxidessuch as MnO₂, V₂ O₅, MoO₃, Pb₃ O₄, Bi₃ O₄, Co₃ O₄, TiO₂, Cr₃ O₈, Cr₂ O₅and LiCoO₂, mixed oxides thereof, metal sulfides, such as TiS₂, CuS andFeS, and mixtures thereof. Among them, MnO₂ is preferred since it has ahigh single-electrode potential and generates a high voltage of about 3V in the cell comprising lithium as the negative electrode. In addition,complex MnO₂ or modified MnO₂, which have been recently developed canachieve good charge-discharge cycle characteristics.

Examples of active materials for the negative electrode are light metalssuch as lithium, potassium, sodium, calcium and magnesium and lithiumalloys such as LiAl, LiIn, LiCd, LiSi, LiGa, etc. Among them, lithiummetal is preferred.

FIG. 2 shows a cross sectional view of a spiral wound cell according tothe present invention. The cell comprises a positive electrode 1, anegative electrode 2, a bag shape separator 3, which wraps the positiveelectrode 1, and a non-aqueous electrolytic solution 4. The electrodes 1and 2 are laminated and spirally wound and then installed in acylindrical stainless steel cell case which acts as the negativeelectrode can. The whole electrodes are immersed in the electrolyticsolution 4.

In addition to the spiral wound cell of FIG. 2, the cell may be in anyform such as a can-type cell, a button shape cell, a coin shape cell orother thin cells.

PREFERRED EMBODIMENTS OF THE INVENTION

The present invention will be illustrated by the following Examples.

EXAMPLE 1

A band shape lithium negative electrode having a thickness of 0.17 mmand a width of 30 mm and a band shape MnO₂ positive electrode having athickness of 0.4 mm and a width of 30 mm which was wrapped in a bagshape separator made of microporous polypropylene were laminated andwound. To each electrode, a lead member was attached. Then, the woundelectrodes were installed in a stainless steel cell case having an outerdiameter of 15mm.

Separately, an electrolytic solution was prepared by dissolving 0.5mole/liter of LiClO₄ and 0.1 mole/liter of LiPF₆ in a mixed solvent ofpropylene carbonate, tetrahydrofuran and 1,2-dimethoxyethane in a volumeratio of 1:1:1, removing water from the solution and then adding 0.5% byvolume of tributylamine. The solution contained less than 50 ppm ofwater.

The electrolytic solution was poured in the cell case which containedthe electrodes.

The opening of the cell case was closed, and the cell was stabilized andaged to obtain a cylindrical spiral wound cell having an outer diameterof 15 mm, a height of 40 mm and a structure shown in FIG. 2.

EXAMPLE 2

In the same manner as in Example 1 but using 0.5% by volume ofN,N-dimethylacetamide in addition to tributylamine, the cell wasproduced.

COMPARATIVE EXAMPLE 1

In the same manner as in Example 1 but using no tributylamine, the cellwas produced.

COMPARATIVE EXAMPLE 2

In the same manner as in Example 1 but using 5% by volume ofN,N-dimethylacetamide in place of tributylamine, the cell was produced.

Each of the cells produced in Examples 1 and 2 and Comparative Examples1 and 2 was stored at 60° C. for 100 days. Every 20 days, the closedcircuit voltage after 0.5 second at 3 A was measured. The results areshown in FIG. 3, in which the curves 1a, 1b, 1c and 1d represent theresults for the cells produced in Examples 1 and 2 and ComparativeExamples 1 and 2, respectively.

As is clear from these results, the addition of tributylamine or acombination of tributylamine and N,N-dimethylacetamide to theelectrolytic solution greatly improved the storage stability of thecells.

With the non-aqueous electrolytic solutions prepared in Examples 1 and 2and Comparative Examples 1 and 2, stabilizing tests were carried out asfollows:

Ten milliliters of the non-aqueous electrolytic solution was charged ina 10 ml vial. In the vial, a lithium metal piece of 1 cm×4 cm was added.The opening of the vial was closed with a polyethlene stopper and sealedwith an aluminum cap. Then, the vial was stored at 80° C. for 10 daysand opened. The surface condition of the lithium piece and color of theelectrolytic solution were observed. The results are shown in Table 1.

                  TABLE 1                                                         ______________________________________                                        Example    Condition of  Color of electro-                                    No.        Li piece surface                                                                            lytic solution                                       ______________________________________                                        1          Gloss         Slightly colored                                     2          Gloss         Transparent                                          Comp. 1    Black colored Brown                                                           over whole                                                                    surfaces                                                           Comp. 2    Slightly colored                                                                            Substantially                                                   over whole    transparent                                                     surfaces                                                           ______________________________________                                    

As understood from the results of Table 1, the non-aqueous solution ofthe present invention can protect the surfaces of the lithium piece andstabilize LiPF₆.

EXAMPLES 3 to 6

In the same manner as in Example 1, but using an additive listed inTable 2, the cell was produced.

The cell had the same storage stability as those produced in Examples 1and 2.

The stabilizing test was carried out in the same manner as in Example 1.The results are shown in Table 2.

                  TABLE 2                                                         ______________________________________                                        Example Additive*.sup.1)                                                                        Condition of Color of electro-                              No.     (vol. %)  Li piece surface                                                                           lytic solution                                 ______________________________________                                        3       THA (1)   Gloss        Slightly colored                               4       TDA (1)   Gloss        Slightly colored                               5       THA (0.5) Gloss        Substantially                                          HMPA (1)               transparent                                    6       TBA (0.5) Gloss        Transparent                                            DEAD (1)                                                              ______________________________________                                         Note:                                                                         *.sup.1) THA: Trihexylamine. TDA: Tridecylamine. TBA: Tributylamine. DEAD     N,NDiethylacetamide.                                                     

EXAMPLES 7 TO 12 AND COMPARATIVE EXAMPLES 3 AND 4

In the same manner as in Example 1 but using an electrolyte and anadditive listed in Table 3, the cell was produced.

The cells produced in the Examples had better storage stability thanthose produced in the Comparative Examples.

The stabilizing test was carried out in the same manner as in Example 1.The results are shown in Table 3.

What is claimed is:
 1. An organic electrolytic solution cell comprisinga positive electrode, a negative electrode which comprises lithium andan electrolytic solution of a lithium salt in a polar solvent whichcontains a compound comprising an organophobic group having at least onestraight chain alkyl group with at least three carbon atoms and anorganophilic group selected from the group consisting of a ketone group,an ether group and an ester group, said organophobic and organophilicgroup being directly connected.
 2. The organic electrolytic solutioncell according to claim 1, wherein at least one hydrogen atom of saidalkyl organophobic group may be substituted by a fluorine atom.
 3. Theorganic electrolytic solution cell according to claim 1, wherein saidalkyl group has at least 4 carbon atoms.
 4. The organic electrolyticsolution cell according to claim 1, wherein said organophilic group is aketone group.
 5. The organic electrolytic solution cell according toclaim 1, wherein said organophilic group is an ether group.
 6. Theorganic electrolytic solution cell according to claim 1, wherein saidorganophilic group is an ester group.
 7. The organic electrolyticsolution cell according to claim 1, wherein said compound comprises oneorganophilic group bonded with at least two organophobic groups.
 8. Theorganic electrolytic solution cell according to claim 1, wherein saidelectrolytic solution contains 0.05 to 5% by volume of said compound. 9.The organic electrolytic solution cell according to claim 8, whereinsaid electrolytic solution contains 0.1 to 1.5% by volume of saidcompound.
 10. The organic electrolytic solution cell according to claim1, wherein said polar solvent includes propylene carbonate.
 11. Theorganic electrolytic solution cell according to claim 1, wherein saidlithium salt contains LiPF₆.
 12. The organic electrolytic solution cellaccording to claim 11, wherein said electrolytic solution furthercontains a stabilizer for said LiPF₆.
 13. The organic electrolyticsolution cell according to claim 12, wherein said stabilizer is at leastone member selected from the group consisting of N,N-dimethylacetamide,N,N-diethylacetamide and 1-methylpiperidone.
 14. The organicelectrolytic solution cell of claim 11, wherein said lithium saltfurther includes LiClO₄.